The invention relates to a process for preparing an improved propylene glycol antifreeze containing silicate which is resistant to gellation of the silicate.
Antifreeze compositions comprising ethylene glycol and a wide variety of corrosion inhibitors are well known in the prior art as evidenced by U.S. Pat. Nos. 3,341,469, 3,337,496, 4,241,014 and 4,664,833. Although prior art patents disclose the use of a wide variety of glycols in antifreeze compositions, such patents primarily deal with and provide working examples for ethylene glycol based antifreeze compositions. Further, the prior art has not generally disclosed any criticality of the mixing order of the corrosion inhibitors to the ethylene glycol. Two exceptions to this are found in U.S. Pat. Nos. 3,282,846 (Scott) and 4,149,985 (Wilson). Scott discloses a method for manufacturing an antifreeze involving the mixing order of ethylene glycol, borate, silicate, and sodium mercaptobenzothiazole. Wilson discloses a method for manufacturing an antifreeze involving the mixing order of ethylene glycol, borate, basic inorganic compound (until a pH of from 9.5 to 10.5) and then silicate.
It has been found that the order of mixing disclosed in the Scott and Wilson patents is not applicable to all types of alkylene glycols and is particularly inapplicable for manufacturing propylene glycol antifreeze compositions. U.S. Pat. No. 5,422,026 (Greaney) discloses a phosphate-free corrosion inhibited antifreeze prepared by admixing an aqueous solution comprising silicate, silicate stabilizer (polysiloxane), molybdate, and a relatively small amount of a basic compound, with a solution of an alkylene glycol, preferably, propylene glycol, and the various corrosion inhibitors typically employed, e.g., mercaptobenzotriazole, borate, nitrate, sebacate, tolyltriazole, and the like. Greaney does not recognize or disclose the need for a high pH in the initial silicate premix in achieving long term silicate stability without gel formation. Applicants' invention overcomes the prior art limitations and provides a process for preparing gel resistant propylene glycol antifreeze compositions containing alkali metal silicate.